Take VOIP Bandwidth

I have a Dsl 256/128 connection plus VOIP.Therefore my ISP gives me 640/448.I own an Alcatel Speedtouch 530 which is connected with a Cisco ATA 186 via ethernet.Speedtouch has two ATM channels (Data and Voice). 256/128 is preserved by the Data channel and the other bandwidth is taken by Voice.My questin is: Is there any way to take the extra bandwidth for the voice to the data channel and give priority to the voice so when i use the telephone the voice channel takes back its bandwidth;

Thanks in advance for your help…